Miscible displacement of petroleum

ABSTRACT

A miscible displacement process for the recovery of petroleum from a petroleum-bearing reservoir is performed in situ by use of a solvent miscible with the petroleum and having a density greater than water followed by an aqueous driving fluid wherein the solvent is introduced in the vapor state into the reservoir or is vaporized within the reservoir by a heated driving fluid and proceeds in a substantially horizontal manner through the reservoir forming a condensation front ahead of the driving fluid.

" lee- 274 SR T197 12 GR 396473224 7:2 United States Patent 1191 1111 3,847,224 Allen et a1. Nov. 12, 1974 41 MISCIBLE DISPLACEMENT 0F 3,131,760 5/1964 Arendt et a1 166/268 P ROLEUM 3,249,157 5/1966 Brigham ct a1. 166/273 3,729,053 4/1973 Froning 166/273 [75] Inventors: Joseph C. Allen, Bellaire; Jack F.

Tate Houston both of Primary Examir'rer-.lames A. Leppink [73] Assigneei Texaco Inc" New York, attorney, Agent, or Firm-Thomas H. Whaley; C. G.

[22] Filed: May 4, 1973 [21] Appl. No.: 357,406 1 1 S CT A miscible displacement process for the recovery of petroleum from a petroleum-bearing reservoir is per- 2 l. g E 5i :2512 formed In s1tu by use of a solvent mlscible Wl1.h the pe- [58] Field "166066475 troleum and having a density greater than water followed by an aqueous driving fluid wherein the solvent [56] References Cited is introduced in the vapor state into the reservoir or is vaporized within the reservoir by a heated driving UNITED STATES PATENTS fluid and proceeds in a substantially horizontal man- :{p ner through the reservoir forming a condensation 1 1 front ahead of the drivin fluid. 2,910,123 10/1959 Elkins et a1. 166/271 g 3,047,063 7/1962 Connally, Jr. et a1. 166/273 9 Claims, 3 Drawing Figures 1 MISCIBLE DIS-PLACEMENT or PETROLEUM BACKGROUND OF THE INVENTION inefficiency of these displacement processes is partly due to the retentive forces of capillarity and interfacial tension. Miscible flooding provides a method for efficiently displacing the petroleum from a reservoir.

In miscible flooding, solvent for the petroleum is introduced into the reservoir and driven through the reservoir. Dissolution of the petroleum by the solvent permits no two phase system between the solvent and the petroleum to exist at the conditions of temperature and pressure existing in the reservoir. Therefore, the retentive forces of capillarity and interfacial tension are nonexistent. These forces decrease the displacement efficiency of a recovery process where the driving fluid or displacing agent and the petroleum exist as two phases in the reservoir.

In a miscible flood process the solvent has the capability of mixing completely with the petroleum in the reservoir. A transition zone is formed at the leading edge of the solvent between the solvent and the petroleum in' which miscibilityexists between the solvent andthe petroleum. Foreconornic reasons the solvent is normally injected as a slug followed by another fluid such as a gas or'an aqueous fluid to drive the solvent slug and the petroleum through the resevoir. In displacement processes in general, theide'al sought after is piston-like displacement. That is, thedisplacing fluids-should'ideally present aflat front to the petroleum in-the reservoir and displace it uniformly through the reservoir. Most miscible solvent slugs are followed by an aqueous fluid to drive them through the reservoir. Moreover, most miscible solvents have heretofore been light hydrocarbons with densities lessthan water. Problems have arisen with such processes, however.

In a vertical miscible flood, for example, using a light hydrocarbon solvant slug followed by water, the water will tend to finger through the less dense solvent due to viscous fingering and gravity segregation, destroying piston-like displacement and resulting in premature breakthrough of the displacing medium water. Further,

there are certain petroleum depositswhichare only partially soluable 'in the prior art solvents. One type of 55 petroleum which is only partially soluble in prior art solvents are the tar'sand oils.

Throughout the world there are various known locations wherein the earth contains large deposits of tar sands. For example, one of the most extensive and best known deposits ofgthis type occurs in the Athabasca district of Alberta, Canada. In the tar sands in such de-- posits, the oiltypically has a density approaching or even greater than that of water. The Athabasca tar sands extend for many miles and occur in varying thick- I ness of up to more than 2100 feet. Although in some places the Athabasca tar sands are disposed practically on the surface of the earth, generally they are located under an overburden which ranges in thickness from a few feet to as much as 1000 or more feet in depth. The tar sands located at these depths constitute one of the worlds largest presently known petroleum deposits. In these sands, the oil content ranges between about and 20 percent by weight, although sands with lesser or greater amounts of oil content are not unusual. Additionally, the sands generally contain small amounts of water in the range of from about 1 to 10 percent by weight.

The oil present in and recoverable from Athabasca tar sandsis usually a rather viscous material ranging in specific gravity from slightly below 1.00 to about 1.04 or somewhat greater. At a typical reservoir temperature, e.g., about 48 F., this oil is immobile, having a viscosity exceeding several thousand centripoises. At higher temperatures, such as temperatures above about 200 F. this oil becomes mobile, with visco'si ties of less than about 343 centipoises, and the tar sands are incompetent. Since this tarry material does not generally command a very high price, particularly when in its crude state, its separation and recovery must involve a minimum of expenditure in order to be economically attractive for commercial practice.

SUMMARY OF THE INVENTION The invention is a process for recovering petroleum from a reservoir by miscible displacement. A solvent more densethan water is introduced into a petroleum reservoir in the vapor state or is vaporized within the reservoir by the heated driving fluid and is driven in a substantially horizontal manner toward a production point by a driving fluid. The solvent cools as it proceeds through the reservoir and a condensing front of solvent forms ahead of the solvent in the vapor state. As the condensed solvent front reaches the production point the solvent injection is stopped and injection of the driving fluid continued.

BRIEF DESCRIPTION OFETHE DRAWINGS DESCRIPTION or THE PREFERRED EMBODIMENTS This invention is an improvement'in the displacement of heavy oil, such as tar "sand oil. The improvement comprises a'horizontal miscible flood with a-solvent for petroleum having a density greater than an aqueous fluid and preferably 'a viscosity less than an aqueous fluid. A fluid having a vapor pressure less than the "solvent is injected concurrent with or following the solvent. Aqueous fluid, as used herein, denotes water and water solutions such as brine. The terms water or'aqueous fluid may also include water thickened with polymers or other chemicals.

7 Although aqueous fluids such as water are the preferred fluids for displacing the solvent through the res ervoir, any fluid having favorable properties may be used. The displacing fluid must beli'ghter than the solvent and preferably more viscous. The -fluid should also be substantially unreactive with the solventa'nd immiscible'with the solvent.

The types of solvents useful in the process of our invention are those which are heavier than and essentially chemically inert to water and have solubility characteristics which enable them to dissolve adequate amounts of petroleum. It is preferred that the solvent have a viscosity less than water. Ideally, the solvent should be miscible with the petroleum so that the interface between the leading edge of the solvent and the petroleum is removed. Examples of specific solvents include but are not limited to carbon disulfide and chlorinated hydrocarbons such as methylene dichloride and carbon tetrachloride. Any solvent more dense than the aqueous driving fluid may be used.

In certain applications carbon disulfide is the preferred solvent because of its unique properties or ease of manufacture and recovery. In the case of tar sand oil, for example, the bitumen is more soluble in carbon disulfide than in other solventsand certain bitumens may only be soluble to any appreciable extent in carbon disulfide. Also, where the recovered crude is to be catalytically treated in a refinery, for example, carbon disulfide is preferred. It is a characteristic of covalently bonded halogens such as those found in halongenated hydrocarbons that they tend to poison some refinery catalysts. Carbon disulfide does not and in addition is quite easily removed from recovered crude by physical separation processes to be reused again, leaving the crude substantially free of carbon disulfide. Carbon disulfide may also have a great economic advantage over halogenated hydrocarbons since it may be manufactured by the reaction between coke (carbon) and sulfur. Coke and sulfur are often found in excess near prolific tar sand deposits such as the Athabasca tar sands of Canada. The use of these materials would be an aid to conservation of the environment.

It it also within the scope of our invention to use as a solvent a blend of carbon disulfide with another component, mutually soluble in carbon disulfide such as a chlorinated hydrocarbon. These materials should also be easily removed from dissolved'tar sand oil by physical separation techniques such as vacuum distillation.

The invention may be more clearly understood by referring to the attached figures which depict a typical embodiment of the process. This embodiment is offered by 'way of example only and is not intended to limit the scope of our invention. Variations of techniques to produce substantially the same results will be obvious to those skilled in the art after reference to the teachings herein.

Referring to FIG. 1, carbon disulfide vapor 14 has been injected via well 11 into a petroleum-bearing reservoir 13. An aqueous fluid 15 hot enough to vaporize carbon disulfide has also been injected into the reservoir 13 via well 11. As the carbon disulfide vapor progresses farther from well 11 its temperature becomes lower and it condenses into a liquid phase 16 and migrates to the bottom of the reservoir due to its density. The water is then forced to partially override this condensed carbon disulfide as shown. FIGS. 2 and 3 depict the process of our invention in later stages. The carbon disulfide vapor continues to condense and form a larger and longer liquid front. The vaporized carbon disulfide invades the entire upper section of the reservoir while the condensed carbon disulfide invades the lower section of the formation. In FIG. 3 the liquid carbon disulfide front has reached the production well 12. Carbon disulfide injection into well 11 may be terminated while hot water injection continues until all carbon disulfide is converted to vapor. Water injection may then be terminated and the reservoir produced by pressure depletion.

An alternate procedure to the above may be used which would eliminate vaporizing the solvent before injection. Liquid carbon disulfide is injected and invades the lower vertical section of the formation. Water hot enough to vaporize the liquid carbon disulfide in the formation is injected and flows toward the production well over the liquid carbon disulfide. The carbon disulfide is thus heated and vaporized. The vaporized carbon disulfide flows upward through the .water zone and segregates above the water. This provides excellent mixing of the hot gaseous solvent, hot water, and reservoir oil. The overall efficiency is increased. As the carbon disulfide vapor flows horizontally it is cooled, condenses and flows downward to occupy the lowermost section of the reservoir. The process then proceeds substantially as in the first described embodiment.

Many variations are possible without departing from the scope of our invention. For example, steam instead of water may be used and alternate slugs of solvent and hot water and/or steam will provide excellent results.

It is known that at certain extreme conditions carbon disulfide will react with steam. For example, at 700 C. steam reacts with carbon disulfide vapor but at 200 C. no reaction occurs. At 210 C. and 300 psig in the presence of an alumina-silica catalyst reaction occured in the vapor state. However, at C. no reaction occured between liquid refluxing water and vaporized carbon disulfide. Therefore, when using steam at high temperatures the possibility of reaction between carbon disulfide is present. To avoid this reaction it is preferred to use a liquid aqueous fluid with carbon disulfide or use another solvent such as the chlorinated hydrocarbons if steam is used. No reaction occurs between the chlorinatd hydrocarbons and water or steam. 4

The process of this invention is operable in a variety of petroleum reservoirs containing petroleum of widely differing gravities. 'On'e preferred embodiment, however, is to recover tar sand oil using a solvent comprising a major amount of carbon disulfide. A particularly preferred embodiment is the recovery of tar sand oil using a solvent slug comprising a major amount of carbon disulfide followed by an aqueous driving fluid. As pointed out previously, it is a characteristic of the bitumen constituents of tar sand oil or petroleum that they are soluble in carbon disulfide and less soluble or insoluble in most other solvents.

A very important advantage of using carbon disulfide is the lack of an emulsification of the separate water and carbon disulfide phases. The phases separate into distinct layers easily separable from each other. This feature is advantageous for many reasons. For example, emulsification within the formation could lead to a reduction in permeability due to what is commonly known as emulsion blockage."The lack of emulsification when carbon disulfide is used prevents this problem from occuring. Also, emulsification could destroy piston-like displacement. Another advantage of the lack of emulsion forming tendency between carbon disulfide and water occurs when the solvent, bitumen and water are produced and separation of the carbon sulfide is desired. Emulsion formation would distinctly hamper these operations.

We claim:

1. A method for recovering petroleum from a reservoir having at least one injection well and one production well penetrating and in communication with the reservoir comprising:

forcing carbon CllSUlfldC substantially horizontally through the reservoir with a driving fluid from the injection well to the production well wherein the carbon disulfide is injected in a vaporized state and condensation of the carbon disulflde takes place before it reaches the production well.

2. The method of claim 1 wherein the driving fluid is an aqueous fluid.

3. The method of claim 2 wherein the aqueous fluid is a liquid.

.4. A method for recovering petroleum from a reservoir having at least one injection well and one production well penetrating and in communication with the reservoir in a substantially horizontal plane wherein carbon disulfide is injected into the reservoir and the reservoir conditions are such that the carbon disulfide would be rendered a liquid at these conditions comprismg:

a. injecting the carbon disulfide into the injection well in a vaporized state, and

b. concurrently injecting with the carbon disulfide a fluid having a vapor pressure lower than carbon disulfide.

5. The method of claim 4 wherein the driving fluid is an aqueous fluid.

6. The method of claim 5 wherein the aqueous fluid is a liquid.

7. A method for recovering petroleum from a reservoir having at least one injection well and one production well penetrating and in communication with the reservoir comprising:

forcing carbon disulfide substantially horizontally through the reservoir with a driving fluid wherein the carbon disulfide is injected in a liquid state and the driving fluid is at such a temperature that the carbon disulfide will be vaporized in the reservoir.

8. A method as in claim 7 wherein the driving fluid is an aqueous fluid at a temperature such that essentially no reaction takes place between the carbon disulflde and the aqueous fluid.

9. A method as in claim 8 wherein the aqueous fluid UNITED STATES PATENT OFFICE CERTIFICATE OF' CORRECTION PATENT N0. 3, 47,224 Q I DATED November 12, 1974 INVENTOR( 1 Joseph C. Allen and Jack -F. Tate it is certified that error appearsin the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2, line 5, for "llO' insert l0--.

Signed and sealed this 20th day bf May 1975.

(SEAL) Attest:

C. MARSHALL DANN RUTH C. MASON Commissioner of Patents Attesting Officer v and Trademarks 

1. A METHOD FOR RECOVERING PETROLEUM FROM A RESERVOIR HAVING AT LEAST ONE INJECTION WELL AND ONE PRODUCTION WELL PENETRATING AND IN COMBINATION WITH THE RESERVOIR COMPRISING: FORCING CARBON DISULFIDE SUBSTANTIALLY HORIZONTALLY THROUGH THE RESERVOIR WITH A DRIVING FLUID FROM THE INJECTION WELL TO THE PRODUCTION WELL WHEREIN THE CARBON DISULFIDE IS INJECTED IN A VAPORIZED STATE AND CONDENSATION OF THE CARBON DISULFIDE TAKES PLACE BEFORE IT REACHES THE PRODUCTION WELL.
 2. The method of claim 1 wherein the driving fluid is an aqueous fluid.
 3. The method of claim 2 wherein the aqueous fluid is a liquid.
 4. A method for recovering petroleum from a reservoir having at least one injection well and one production well penetrating and in communication with the reservoir in a substantially horizontal plane wherein carbon disulfide is injected into the reservoir and the reservoir conditions are such that the carbon disulfide would be rendered a liquid at these conditions comprising: a. injecting the carbon disulfide into the injection well in a vaporized state, and b. concurrently injecting with the carbon disulfide a fluid having a vapor pressure lower than carbon disulfide.
 5. The method of claim 4 wherein the driving fluid is an aqueous fluid.
 6. The method of claim 5 wherein the aqueous fluid is a liquid.
 7. A method for recovering petroleum from a reservoir having at least one injection well and one production well penetrating and in communication with the reservoir comprising: forcing carbon disulfide substantially horizontally through the reservoir with a driving fluid wherein the carbon disulfide is injected in a liquid state and the driving fluid is at such a temperature that the carbon disulfide will be vaporized in the reservoir.
 8. A method as in claim 7 wherein the driving fluid is an aqueous fluid at a temperature such that essentially no reaction takes place between the carbon disulfide and the aqueous fluid.
 9. A method as in claim 8 wherein the aqueous fluid is at 212* F. or below. 